66 kV is the workhorse voltage class of sub-transmission networks across Egypt, the Middle East, Africa, and parts of Southeast Asia. It sits between medium-voltage distribution and high-voltage transmission, and it carries a disproportionate share of the load in fast-growing grid systems. For procurement engineers and project developers, understanding what makes a 66 kV transformer technically different from its 11 kV or 220 kV counterparts is essential for specifying the right unit.

The 66 kV Insulation System: More Than Just a Number

In IEC terminology, a 66 kV system corresponds to a highest voltage for equipment (Um) of 72.5 kV (or 145 kV for 132 kV systems). This places the 66 kV class in a critical transition zone in IEC 60076-3:

For Um ≤ 72.5 kV, switching impulse tests are not mandatory

For Um > 72.5 kV, switching impulse levels are defined and switching impulse tests become applicable

A 72.5 kV-rated transformer sits right at the boundary. This means the lightning impulse withstand level and power frequency withstand level carry most of the dielectric design burden. For a typical 66 kV winding, the specified insulation levels are:

Parameter Typical Value (66 kV Class)

Highest voltage for equipment (Um) 72.5 kV

Rated lightning impulse withstand (LI) 325 kVp

Rated power frequency withstand (PFWV) 140 kVrms

These values are consistent with IEC 60076-3 test level tables and are widely referenced in utility specifications for 66 kV power transformers.

Design Considerations Specific to 66 kV Class

Core material. High-performance 66 kV transformers should use prime grade CRGO M4 grade or better (Hi-B grain-oriented steel), with a building factor in the range of 1.05–1.15. Lower thickness grades reduce eddy current losses. Step-lap joints and boltless yoke construction minimize noise and vibration, which is increasingly important for urban substations in Cairo, Alexandria, and other densely populated areas.

Short-circuit withstand. IEC 60076-5 governs the ability to withstand short-circuit. The test must account for the network short-circuit impedance, which for 66 kV systems in Egypt's EETC grid can be substantial given the interconnection with 220 kV and 500 kV transmission rings. 10% (with tolerance −0% to +10%). Designs must be verified for dynamic short-circuit effects, with post-test reactance increase kept below 1% for transformers above 100 MVA (a stricter criterion than the general requirement).

Cooling class. The majority of 66 kV sub-transmission transformers in the 16–40 MVA range use ONAN/ONAF cooling, which provides a 1.5–2× capacity increase when fans are activated. For higher ratings, OFAF or OFWF may be specified.

Tap changer. On-load tap changers (OLTC) are standard for 66 kV sub-transmission transformers, typically with ±10% tap range in 1.25% or 2.5% steps. Conservator design for the OLTC — common or separate — is a vendor-specific decision that should be clearly documented in the technical proposal.

Factory Testing Protocol for 66 kV Transformers

Per IEC 60076-3, the following tests are routine for 66 kV class transformers:

Test Standard Reference

Winding resistance measurement IEC 60076-1

Voltage ratio and vector group IEC 60076-1

Short-circuit impedance and load loss IEC 60076-1

No-load loss and Excitation current IEC 60076-1

Lightning impulse (LI) test IEC 60076-3

Induced overvoltage with PD measurement IEC 60076-3

Applied voltage test IEC 60076-3

Insulation resistance IEC 60076-1

For the induced overvoltage test with partial discharge measurement, the test procedure per IEC 60076-3:2013 Edition 3 requires measurement at 1.3×Um/√3 for 30 seconds and 1.1×Um/√3 for 60 minutes. Partial discharge measurement is a routine test for all windings when Um > 72.5 kV, and is strongly recommended for 66 kV transformers serving critical loads.

66 kV Transformers in Egypt's Grid

Egypt's EETC operates a grid structure of 11/22 kV at medium voltage and 66/220/500 kV at transmission, with 66/132 kV serving as sub-transmission. EETC specifications follow IEC 60076 with a 50°C ambient design and 1,500 m altitude correction for Upper Egypt. Coastal zones like Alexandria require silicone-rubber bushings rated for marine salt fog and copper windings with optional ester-fluid insulation for indoor units.

Recent procurement activity confirms sustained demand. EETC has contracted for 51 transformers rated 11–66 kV, 40 MVA across Cairo, Canal, Alexandria, West Delta, Middle Egypt, Upper Egypt, and Delta zones, and for three 175 MVA transformers rated 11/66/220 kV for Sadat Main and Tahrir Badr substations.

How Hualai Serves 66 kV Projects

Jiangsu Hualai Electric Technology Co., Ltd. manufactures 66 kV class oil-immersed power transformers from 16 MVA to 63 MVA at 66/11 kV, 66/22 kV, and 66/33 kV configurations. Our designs are:

IEC 60076-3 compliant, with full routine and type test certification

Engineered for 50°C ambient and high-altitude correction for Upper Egypt and Sahel conditions

Available with silicone-rubber bushings for coastal and high-pollution environments

Equipped with OLTC (common or separate conservator per customer preference)

Every unit undergoes complete factory acceptance testing with witnessed FAT available at our Jiangsu facility. Full type test reports from ISO/IEC 17025-accredited laboratories are provided with each shipment.

📩 For 66 kV transformer specifications or project inquiries, contact hualaipower@gmail.com or WhatsApp: +86 18852130008.